Throughout the Democratic primary race, Barack Obama has cast himself as an underdog trying to wrest the nomination from the grip of the party establishment, which he contends is partial to rival Hillary Rodham Clinton.

But it is Obama, a first-term Illinois senator running against the conventions of Washington, who is increasingly benefiting from institutional support — bolstering his campaign during a rough month when he lost two key primaries and faced questions about his spiritual mentor.

Obama on Monday picked up the endorsement of yet another U.S. senator, Amy Klobuchar of Minnesota, the latest in a slew of superdelegates to throw weight behind him. A group of as many as seven U.S. House members from North Carolina, which holds a key primary on May 6, is poised to announce its support for Obama, The Wall Street Journal reported.

Clinton still leads Obama among committed superdelegates 250 to 217, but Obama has been steadily closing the gap. Since Super Tuesday on Feb. 5, 64 superdelegates have announced their support for Obama, compared with nine for Clinton. That trend is increasing pressure on Clinton, who trails Obama in two other important benchmarks — total delegates and overall popular vote — to consider stepping aside.

What has been striking over the past month is that Obama has racked up key endorsements during a relatively turbulent period in his candidacy. The endorsements — Gov. Bill Richardson of New Mexico and Sen. Bob Casey of Pennsylvania are two of the most influential — have given him a strong underpinning after Clinton's big wins in the March 4 Ohio and Texas primaries, and amid bruising coverage of his relationship with his controversial former pastor, the Rev. Jeremiah A. Wright Jr.

"It's pretty clear that in the last couple weeks the Obama argument seems to have been more persuasive with those superdelegates than the Clinton argument, even in the face of the questions that were raised about Obama," said Mark Mellman, a Democratic strategist who worked on Sen. John F. Kerry's 2004 presidential campaign but is neutral this election. "Whether that's going to continue as we get new primary results from other states, that remains to be seen."

Democratic strategist Chris Lehane, who is supporting Clinton, said: "The issue is, does it become a cascade? I think right now it has not reached that level."

Lehane said that Clinton is, however, facing impatience among many in the party who want to settle on a nominee and start focusing on the presumptive Republican nominee, Sen. John McCain of Arizona.

Of the nearly 800 superdelegates, roughly 330 remain undecided. Because neither Obama nor Clinton can reach the 2,024-delegate threshold to win the nomination without superdelegates, that group of 330 will almost certainly determine who will represent the party against McCain.